The present invention relates to a thermal printing apparatus which generates printed images in a variety of patterns on sheet material. More particularly, the apparatus has a thermal printhead with a plurality of heating elements energized by a stabilized source of power.
Printing apparatuses utilizing thermal printheads are well known, and because of their flexibility they are often utilized in printers which produce a wide variety of printed images, particularly images that are defined in stored or transmitted digital data programs. One such printing apparatus that is utilized for generating signs, designs, characters and numerous other graphic images on a strip of sheet material is disclosed in U.S. patent application 08/007,662, filed Jan. 22, 1993, which has the same assignee as the present invention. The referenced apparatus derives digital data defining the graphic image from a stored printing program and converts that data into multi-colored images.
In order to provide clear images with high resolution, the printheads are constructed with densely packed heating elements arranged in a linear array for selective energization and generation of printed images in small pixels. One such printing head has a linear array of heating elements approximately 12 inches long with elements arranged at a density of 300 per inch.
With a printhead having a large number of heating elements, a regulated power supply is needed and the power supply must have the capacity to energize all of the heating elements simultaneously or substantially simultaneously. Such energization can, however, cause significant load demands and if the image being printed has intermittent printing, that is a sequence of alternate inked and non-inked spots in the direction of printing, the regulating circuits of the power supply may not be able to respond to the load cycling between peak power and zero or low power demands of the printhead. As a consequence of load cycling and inductive impedance in the circuitry between the power supply and the printhead, the printed image may fade and intensify due to current surges that arise as transients in the circuits supplying power to the head. Such transients distort the printed image and cause it to depart from the digital data in the program that defines the image.
A number of known solutions for improving the response of the printhead include increasing the capacity of the power supply. Naturally, however, practical considerations limit the effectiveness of such a solution since the power supply is typically operating at less than 50% capacity.
Another solution which is incorporated in existing drivers for printheads is phased energization of the heating elements in the head. For example, a printhead which is 12 inches long may be divided into four sections which are triggered or enabled by four different strobe or clock signals so that the heating elements in each section are not turned on simultaneously. Instead each section is energized at intervals separated by time increments in the order of milliseconds which will not be perceptible in the printed image. Thus, sudden load demands on the power supply are distributed over a longer period of time.
In spite of the techniques employed to date in minimizing transients, problems involved with power supply to printheads still tend to arise at high printing speeds when the printed image requires substantial numbers of the heating elements in the head to be energized or de-energized simultaneously.
It is accordingly a general object of the present invention to alleviate difficulties that arise with transients in the power supply circuits for a thermal printhead in order to improve the quality and accuracy of the resulting image.